The present application relates to wireless communication devices. More particularly, the present application relates to a wireless communication device capable of controlling switching of power modes, a method of controlling the switching of the power modes, and a program causing a computer to perform the method.
Wireless communication devices, typified by ones conforming to IEEE 802.11 and ones employing Bluetooth, have been mounted on many mobile devices in these days and it becomes important to save power in the wireless communication devices. Many existing wireless communication devices have an intermittent mode (also referred to as a power save mode), in addition to a normal mode in which the transmission and reception is constantly enabled, as power modes. The intermittent mode intermittently has a state in which no transmission and reception is performed in order to reduce the power consumption. However, although the intermnittent mode contributes to reduction in power consumption, access control in the intermittent mode is complicated because it is necessary to switch the states of the transmission and reception operation while keeping the consistency of the transmission and reception timings between the wireless communication devices. In addition, a communication delay possibly occurs due to the intermittent mode in real-time communication, for example, in audio communication or stream communication.
In a typical infrastructure network, such as a basic service set (BSS), in which a point coordinator (control station) is provided to achieve centralized control of communication, the point coordinator centrally manages the states of the transmission and reception operations of other wireless communication devices (stations) to realize the intermittent mode while keeping the consistency of the transmission and reception timings between the point coordinator and the other wireless communication devices. For example, a wireless communication system in which a point coordinator transmits an invocation instructing signal to each station to cancel a sleep state of the station is proposed in, for example, Japanese Unexamined Patent Application Publication No. 11-266254 (FIG. 9).
In contrast, in autonomous decentralized wireless networks, it is necessary to perform the access control in consideration of the states of transmission and reception operations of neighbor wireless communication devices (neighbor stations) when each terminal (including the point coordinator) autonomously switches the states of the transmission and reception operations. The autonomous decentralized wireless networks include ad hoc networks in which terminals (wireless communication devices) perform autonomous decentralized communication without an intervening point coordinator and networks, such as extended service sets (ESSs) including wireless distribution systems (WDSs), in which point coordinators are connected to each other by wireless communication. For example, each terminal communicates with a neighbor station while determining (1) whether the destination terminal is in the normal mode or in the intermittent mode, (2) the state of the transmission and reception operation if the destination terminal is in the intermittent mode, and (3) whether it is necessary to submit a request for switching from the intermittent mode to the normal mode to the destination terminal. Accordingly, it is necessary for each terminal to hold information concerning the states of transmission and reception operations of all the neighbor stations in the autonomous decentralized wireless networks. Consequently, the access control in each terminal is complicated in the autonomous decentralized wireless networks, compared with the infrastructure networks in which it is sufficient for each terminal to acquire information concerning the point coordinator.
In particular, the intermittent mode in the autonomous decentralized wireless networks possibly has a great effect on transmission and reception of broadcast packets. In the transmission of the broadcast packets, it is necessary for all the terminals, serving as the neighbor stations, to be in a reception enabled state. Since each terminal autonomously switches the states of transmission and reception operation, it is necessary to determine timings at which the broadcast packets are to be transmitted. In order to resolve this problem, for example, a common time segment in which all the terminals are in a transmission-reception enabled state is set in advance or the broadcast packets are expanded into unicast packets that are transmitted at timings at which the destination terminals are moved to the reception enabled state.
However, in the former case, since the terminals simultaneously attempt to transmit the broadcast packets in the time segment and the broadcast packets are not generally re-transmitted, the number of non-transmitted broadcast packets possibly increases.
In the latter case, since the broadcast packets are transmitted only to the terminals determined as the neighbor stations, it is necessary for each terminal to hold information concerning the transmission and reception states of all the neighbor stations. In addition, when a mesh network, typified by a sensor network, is built by using a multi-hop technology, the neighbor stations should normally receive the broadcast packets distributed by flooding, such as route request (RREG) packets used for searching for a route from the transmission station to the reception station, at the same timing. However, a large delay occurs in the time when the RREG packets are received depending on the timings of the transmission of the unicast packets and the delay can result in formation of an unexpected route. Furthermore, since it is necessary to transmit and receive the unicast packets of a number corresponding to the number of the neighbor stations, instead of one broadcast packet, the effective utilization ratio of channels is possibly reduced as the neighbor stations is increased in number.
It is desirable to perform forced switching of power modes of neighbor wireless communication devices in an autonomous decentralized wireless network.